Unwanted voltage surges have long been a critical problem to circuit designers of industrial and home electrical systems. Surges generated by load switching are often repetitive and range as high as 2,500 V. Lightning generated surges can range up to or over 50,000 V.
It is known that ZnO, when mixed with certain additives and sintered into pellets, can exhibit nonlinear V-I characteristics, as taught, for example, by Gupta et al., in U.S. Pat. No. 4,094,061. These modified ZnO compositions are candidate materials for nonlinear lightning arrester components and nonlinear resistor applications. Such devices can have nonlinearity, now generally believed to be due to an electrical barrier at the grain boundary between the grains of ZnO, i.e., completely due to electrical phenomenon within the bulk of the body.
The ZnO nonlinear devices have been made by mixing the additives with ZnO powder, and then pressing and sintering at appropriate pressures and temperatures, to form the resistor body. A wide variety of additives has been used. Gupta et al., for example, in U.S. Pat. No. 4,094,061, used 95 mol % ZnO, and 1 mole % each of Bi.sub.2 O.sub.3, CoO, MnO, Cr.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 ; while mentioning that other additives, such as TiO.sub.2, SnO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, and at least a dozen others may also be used. The components were added to water with binder, spray dried, pressed, heated to decompose the binder, and then sintered to form the resistor body.
Other mixtures have omitted various additives, for example, Masuyama et al., in U.S. Pat. No. 3,642,664 omitted certain starting additives, such as Al.sub.2 O.sub.3 and B.sub.2 O.sub.3 ; using batch starting mixtures containing about 85 to 99.95 mole % ZnO with 0.05 to 1.5 mole % of at least one of KF, NaF, LiF, CuF.sub.2, CoF.sub.2, and the like. Masuyama et al., in U.S. Pat. No. 3,760,318, omitted certain additives, such as Al.sub.2 O.sub.3, B.sub.2 O.sub.3, KF, NaF, and LiF; making resistor bodies from ZnO, Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, SnO.sub.2 and the like. These resistor bodies, after sintering, were coated with a paste containing a Li or a Na oxide, carbonate, nitrate, sulfate, fluoride, imide or oxychloride and possibly B.sub.2 O.sub.3, SiO.sub.2, BaO and PbO. The paste was then fired at about 800.degree. C. attempting to cause Li or Na ion diffusion into the surface of the sintered resistor body. Iga et al., in U.S. Pat. No. 3,903,226, also omitted certain additives, such as Al.sub.2 O.sub.3 ; using mixtures containing about 95 to 97 mole % ZnO, with possible minor amounts of other additives, such as 0.02 to 5.0 mole % B.sub.2 O.sub.3. Similarly, Matsuura et al., in U.S. Pat. No. 3,863,193, omitted Al.sub.2 O.sub.3 ; using mixtures containing about 97 mole % ZnO, with possible minor amounts of additives, such as 0.01 to 5.0 mole % B.sub.2 O.sub.3.
Nagasawa et al., in U.S. Pat. No. 4,045,374, on the other hand, omitted B.sub.2 O.sub.3 ; using mixtures of ZnO with minor amounts of additives such as Al.sub.2 O.sub.3. Masuyama et al., in U.S. Pat. No. 3,663,458, taught use of Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and other additive oxides; in amounts of about 0.5 mole % each, in a mixture containing 80 to 99 mole % ZnO and up to 10 mole % Bi.sub.2 O.sub.3, omitting KF, NaF, LiF and the like compounds.
Since 1976, attempts have been made to develop nonlinear resistors for high voltage-high energy absorption applications, such as series capacitor protectors and gapless lightning arrestors. Standard nonlinear resistors exhibit marginal nonlinearity and voltage stability in these new high voltage-high energy applications.
Nonlinearity is a measure of ability to be near insulating at low voltage and conducting at high voltage. Stability is a measure of the ability of the resistor to retain its initial current-voltage characteristics after operating for a substantial period. If the stability is marginal, the resistive current will creep upward over a long time period resulting in excessive heat generation which would be detrimental to operation of these new high voltage-high energy devices.
Fishman et al., in U.S. Pat. No. 3,928,245, addressed problems of stability in metal oxide voltage variable resistors. The improvement there, resulted from using a mixture of 96 mole % ZnO, 0.5 mole % Bi.sub.2 O.sub.3, 0.5 mole % MnO.sub.2, 1.0 mole % Sb.sub.2 O.sub.3, 0.1 mole % BaCO.sub.3, 0.1 mole % B.sub.2 O.sub.3 and 0.25 mole % SiO.sub.2, with omission of Al.sub.2 O.sub.3 and Co.sub.3 O.sub.4. This mixture was pressed into a flat disc, sintered, covered on both faces with a contact layer of silver and insulated about its perimeter to prevent flashover. One or more of these discs could be incorporated into a surge arrestor assembly. While this mixture provided improved stability, even less of a percent change from initial current vs. time of operation is needed for modern high voltage-high energy devices.